It is known in the art to use screws and blades as fastening parts, and in particular the so-called spiral blades. Screws provide an excellent grip in the axial direction, a moderate grip in the radial direction (perpendicular to the screw axis), and almost no grip against rotation around the screw axis. Opposing blades provide a very good grip in radial direction as well as against rotation around the blade axis, but almost no grip in the axial direction.
It is also well-known to use intramedullary nails in the healing process of fractured or broken bones in the jaw or the infra-cervical area (see European document EP-A-257 118). The intramedullary nails comprise a femur nail with bone screws running crosswise to it. The femur nail is relatively resistant to bending and fills up almost the entire intramedullary area of a bone. The femur nail contains several cross holes. By using bone screws (i.e., “locks”) on both sides of the fracture, the femur nail is firmly embodied in the bone. Upon installation of a intramedullary nail the bone becomes strong again within a short time. The proximal area of the femur nail is equipped with a diagonal hole, through which femur neck screws are passed. A twist lock of the femur neck screw is provided in the diagonal perforation, permitting the axial movement of the femur neck screw. This twist lock is achieved by the action of bolting device nails on the femur neck screw in the diagonal perforation area, forming several parallel axes within equidistant slots in a circumferential direction, into which the proximal end femur nails are inserted, themselves extending to these coaxes. The slots permit axial movement of the femur neck screw (transverse to the femur nail). The lock nail intervening in each of the slots prevents inadvertent rotation of the femur neck screw.
While the bone heals, a so-called narrowing frequently occurs whereby the bone shortens within the fracture area. If the femur neck screw does not stop this shortening, there may be a danger of the femur neck screw breaking through the femur head or of the fracture becoming unstable.
There exist various other constructional possibilities to secure the femur neck screw in the direction of rotation and to permit an axial movement at the same time. Such a femur neck screw is thus secured against rotation opposite the femur nail, but the proximal bone fragment can still rotate inversely to the femur neck screw around its axis.
A solution is known from the U.S. Patent Publication No. US2002/0045900 (Harder et al.), where a femur neck screw has two diametrically opposed longitudinal slots into which an essentially u-shaped locking element is pushed or driven in. This solution represents a combination of screws and blades and unites the desired characteristics of both fastening parts. However, the height of the branches of the locking element is relatively small, since they are governed by the femur nail only in the femur neck screw slots, and thus their effect as blades is limited. The branches of the locking element spread themselves along the depth of the longitudinal slots reducing laterally to the screw point. The locking element only serves to prevent the projection of bone fragments from the femur neck screw. The turning lock of the femur neck screw in the femur nail takes place in a way similar to the European document EP-A-257 118 by means of a bolting device nail at the proximal end of the femur nails, inducing the need for the surgeon to work on both sides when inserting and locking the femur neck screw.
The problem of rotation is fixed by the implant system described in WO-A-01/739679 through means of two proximal fixation screws. However, such an operation is more costly and there exists a problem that with small jaw necks the two screws will hardly find any place or none at all to be secured to.
Another system uses a spiral blade as the only proximal locking element, which is secured around the axis by a rotation preventing nail. The proximal bone fragment cannot rotate anymore around the axis of the blade. In practice this means that, owing to its larger bearing surface, the blade cannot cut through the bone “laterally” under load as is the case with other screws. It is, however, unfortunate that the spiral blade does not support the femur head sufficiently in the axial direction.
The spiral blade and the screw of U.S. Patent Publication No. US2002/0045900, as mentioned earlier, must be secured against rotation around the femur nail by an additional element at the proximal end of the femur nails. A particular problem of the femur neck screw US2002/0045900 is that there is reduced sliding ability of the screw/locking element along the screw axis during the drilling in of the femur nails, since upon sinking the branches of the u-shaped locking element are spread by the nail against slots of smaller depth in the screw, and they may block themselves during the drilling in of the femur nail.